1. Field of the Invention
The present invention relates generally to an underwater sounding technique, and more particularly to a method and apparatus with wide receiving dynamic range for processing signals from various underwater targets.
2. Description of the Related Art
Acoustic sounding apparatuses, such as echo sounders and scanning sonar, are typically used to detect underwater objects such as fish, seabed, etc., and to evaluate properties of underwater objects, such as length of fish, size of seabed rocks, etc. Such sounding apparatuses are typically installed on a ship, from where they transmit acoustic signals beneath the ship and into the water, to search for underwater objects and receive echo signals from them. The echo signals received from underwater objects are then processed to determine properties of the underwater objects that generated the echo signals.
Typical/conventional underwater sounding apparatuses include two or more receiving-amplifiers with different gains, that amplify echo signals received from underwater targets. For a given echo signal received from a target, a proper receiving-amplifier is selected from among, for example, a low-gain amplifier and a high-gain amplifier. The receiving-amplifier is selected based on the distance to the target, or on the time interval of travel of an echo signal from the target to a source such as a ship. The distance to a target is determined based on the time it takes for a sound pulse from a source such as a ship or a submarine, to bounce off a target and return as an echo signal to the source. Typically, a low-gain amplifier is selected for strong echo signals, such as signals originating at close-by targets, while a high-gain amplifier is selected for weak echo signals, such as signals originating at far away targets. Typical/conventional sounding technology uses Time Variable Gain Control (TVG) to compensate for underwater propagation loss of echo signals. Typically, to detect a given underwater target, a TVG curve suitable for the target is selected.
While the typical/conventional underwater sounding systems can work when the TVG curve selected for a target corresponds to the strength of the echo signal reflected by that target, the typical/conventional underwater sounding systems encounter challenges and are ineffective for mixed echo signals that are reflected by two or more targets of different target strengths, such as, for example, a small fish together with a big fish and a seabed area, as the TVG curves selected for one target may not be suitable for the other targets.
For example, if an echo signal reflected by a target with a small target strength, such as a small fish, is amplified by a low-gain receiving amplifier and coupled with a TVG curve appropriate for larger objects, the signal-to-noise ratio of the signal becomes unacceptably low. And if an echo signal reflected by a target with a large target strength, such as the seabed, is amplified by a high-gain receiving amplifier and coupled with a TVG curve suitable for small targets, the signal level saturates. Hence, the typical/conventional technology is not effective for target detection for a plurality of targets of different strengths, because the typical/conventional technology gives inaccurate or noisy results.
Disclosed embodiments of this application address these and other issues by using underwater sounding methods and apparatuses with a wide receiving dynamic range for processing underwater signals. The methods and apparatuses can receive and process signals associated with one target among two or more targets of different strengths, or with a plurality of targets of different strengths. The methods and apparatuses process signals received from underwater objects, output signals without switching errors or discontinuous points, and provide highly accurate measurements for underwater objects. The methods and apparatuses can be applied to other signals besides acoustic signals, originating in other media besides a water medium.